Silver halide emulsions are generally prepared by precipitating silver halide grains in a hydrophilic colloid, such as gelatin, by the reaction of a silver salt and a halide salt. The thus-formed emulsion is then physically ripened (grain-growing), washed, to remove the soluble salts from the precipitation step and then chemically sensitized, most often with gold and sulfur, to enhance sensitivity to incident light, and then spectrally sensitized to a particular region of the spectrum.
While the above-described sensitization, first chemical, followed by spectral, is the most common sequence of steps employed, it is known in the art to add spectral sensitizing dyes to the emulsion prior to chemical sensitization. For example, U.S. Pat. No. 3,628,960, issued Dec. 21, 1971 (Phillippaerts), discloses adding the sensitizing dye during preparation of the emulsion and can even be added with one or more of the ingredients used in the formation of the grains.
Similarly, U.S. Pat. No. 4,225,669, issued Sep. 30, 1980 (Locher) discloses adding the spectral sensitizing dye after nucleation is complete and before completion of silver halide precipitation. U.S. Pat. No. 4,828,972, issued May 9, 1989 (Ihama), is directed to a method for manufacturing a silver halide emulsion wherein spectral sensitizing dye is added during preparation of the emulsion prior to the desalting step.
It is also known in the art to add thiocyanates to emulsions at various stages of preparation. For example, U.S. Pat. No. 2,222,264, issued Nov. 19, 1940 (Nietz), is directed to the incorporation of metal and/or ammonium thiocyanates during precipitation, during the first digestion, or during the melting out and the second digestion. Increased sensitivity throughout the entire region of sensitivity is obtained.
U.S. Pat. No. 2,448,060, issued Aug. 31, 1948 (Smith), is directed to incorporating sulfur sensitizers at any stage of the preparation of the emulsion. For example, Example 1 discloses the addition of sodium thiocyanate after precipitation of the silver halide but prior to digestion.
U.S. Pat. No. 3,320,069, issued May 16, 1967 (Illingsworth), is directed to the preparation of silver halide emulsions wherein a water-soluble thiocyanate is supplied to the silver halide after the dispersion is formed but prior to it being washed.
U.S. Pat. No. 4,439,520, issued Mar. 27, 1984 (Kofron), discloses a method for preparing tabular grain emulsions (e.g., col. 67, 1. 44, et. seq.) wherein digestion of the grains takes place in the presence of sodium thiocyanate. After said digestion the emulsion was washed which removed the sodium thiocyanate. Spectral sensitization and then chemical sensitization was carried out on the washed emulsion.